Theories of auditory word recognition posit multiple stages of mental representation to describe the transformation of speech into words; the final stage is the lexicon, whose operation and functions differ across theories. The proposed research examines three issues concerning the function of the lexicon in word recognition, testing predictions of classes of theories that are divided on each issue: (1) whether or not lexical knowledge is used to facilitate speech perception; (2) which portion of a spoken word first makes contact with its representation in the lexicon; (3) the process by which a lexical entry is selected that best matches the speech signal. Experimental manipulations are designed to reveal the operation of the lexicon. Stimuli will consist of words and sentences, and the acoustic, phonological, and semantic characteristics of words will be varied. Multiple experimental tasks, such as phoneme monitoring, phoneme identification, and cross-modal priming, will be used to probe processing at different stages in the recognition system and to provide converging evidence on the issues that will be studied. Specialized computer hardware, software, and audio equipment will control stimulus construction, presentation and data collection. The findings should contribute to an understanding of the role of the lexicon in word recognition.